Cleans up stance handling.

Now that dislocation no longer implicates !is_usable(), we can tidy up stance_damage calculation.
Also fixes stance being processed every tick except for the fourth, instead of the inverse

code/modules/mob/living/carbon/human/human_organs.dm

@@ -68,7 +68,7 @@
 /mob/living/carbon/human/proc/handle_stance()
 	// Don't need to process any of this if they aren't standing anyways
 	// unless their stance is damaged, and we want to check if they should stay down
-	if (!stance_damage && (lying || resting) && (life_tick % 4) == 0)
+	if (!stance_damage && (lying || resting) && (life_tick % 4) != 0)
 		return
 
 	stance_damage = 0
@@ -80,7 +80,7 @@
 	var/limb_pain
 	for(var/limb_tag in list("l_leg","r_leg","l_foot","r_foot"))
 		var/obj/item/organ/external/E = organs_by_name[limb_tag]
-		if(!E || (E.status & (ORGAN_MUTATED|ORGAN_DEAD)) || E.is_stump()) //should just be !E.is_usable() here but dislocation screws that up.
+		if(!E || !E.is_usable())
 			stance_damage += 2 // let it fail even if just foot&leg
 		else if (E.is_malfunctioning())
 			//malfunctioning only happens intermittently so treat it as a missing limb when it procs
@@ -93,7 +93,7 @@
 				spark_system.start()
 				spawn(10)
 					qdel(spark_system)
-		else if (E.is_broken() || !E.is_usable())
+		else if (E.is_broken())
 			stance_damage += 1
 		else if (E.is_dislocated())
 			stance_damage += 0.5